I N F O R M A T I O N N O T E

Forests, Carbon andClimate Change: the UK Contribution

231 Corstorphine RoadEdinburghEH12 7AT

www.forestry.gov.uk

J U N E 2 0 0 3

FCIN48

B Y M A R K B R O A D M E A D O W A N D R O B E R T M A T T H E W S O F F O R E S T R E S E A R C H

FORESTS, CARBON DIOXIDEAND GLOBAL CLIMATE CHANGE

The global carbon cycle

A discussion of UK forests and carbon-related issues muststart with an overview of global climate change andgreenhouse gas (GHG) emissions. Carbon dioxide (CO2)is one of the so-called ‘greenhouse gases’ which areresponsible for absorbing energy from the sun, leading towarming of the earth’s atmosphere – the ‘greenhouse effect’.Many GHGs occur naturally in the atmosphere and theirpresence is important for ensuring that the global climateis warm enough to support life.

In the absence of GHGs, the planet would be 30ºC coolerand most life in its present form would not exist.However, there is consensus among scientists that theworld’s climate is changing because the concentrations ofGHGs in the atmosphere are rising. Carbon dioxide is themost important contributor to this enhanced greenhouseeffect. During the 1990s, the atmospheric concentration ofCO2 increased by about 1.5 parts per million (ppm) peryear, continuing an upward trend which has seen a pre-industrial concentration of ~_ 280 ppm rise to the currentvalue of ~_ 370 ppm. Underlying this overall increase is acomplex pattern of exchanges of carbon between theatmosphere, the oceans, terrestrial vegetation (includingforests) and fossil fuel reserves, summarised in Figure 2 asa simplified global carbon budget.

Forests and the global carbon cycle

The rate at which greenhouse gases are being released intothe atmosphere has increased mainly due to the burning offossil fuels for both domestic and industrial purposes, butalso as a result of land clearance and deforestation. Allplant material contains carbon (normally around 50% ofdry weight), and burning or decomposition of clearedvegetation releases it to the atmosphere, mainly in theform of CO2. Plants and particularly trees, because oftheir large biomass per unit area of land, continue tomake an important contribution to the global carboncycle. While deforestation is estimated to have released anadditional 1.6 GtC1 per year into the atmosphere during

SUMMARY

This Information Note outlines how forests in the UK contribute to the carbon cycle on both a local and a global scale. Itexplains the key terms that are used in discussions of the part played by forests and carbon in global warming and presentssome of the facts and figures behind the many complex issues surrounding the subject. The roles played by growing treesand mature woodland, forest soils, harvested wood and wood products in use are discussed and compared. The statisticsand supporting interpretation identify the current and potential contribution of UK woodland in mitigating globalwarming. Information is also provided on carbon management in forests which is relevant to woodland managers andforestry practitioners as well as to those considering involvement in carbon trading and carbon-neutrality schemes.

1

Figure 1

Woodland has an important role to play in the global carbon budget.

11 GtC (1 gigatonne C) = 1000 million tonnes carbon = 1015 g carbon;1 MtC (1 megatonne C) = 1 million tonnes carbon = 1012 g carbon.

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2

T H E K E Y C O M P O N E N T S O F

F O R E S T C A R B O N

W h e n c o nsi d eri n g t h e c o ntri b uti o n m a d e b y w o o dl a n d t o

t h e c ar b o n b al a n c e at a n y s c al e, t h e r at e at w hi c h C O2 i s

r e m o v e d fr o m t h e at m os p h er e a n d/ or t h e q u a ntit y of

c ar b o n r et ai n e d i n t h e w o o dl a n d as a r es er v oir ( als o

k n o w n as a c ar b o n p o ol) s h o ul d b e ass ess e d. T his

ass ess m e nt s h o ul d c o nsi d er a n u m b er of p o ols i n a d diti o n

t o t h e a b o v e- gr o u n d, visi bl e c o m p o n e nts of t h e f or est.

W hil e it is e as y t o a p pr e ci at e t h at t h e r o ots of tr e es

c o nt ai n a si g nifi c a nt b ut hi d d e n pr o p orti o n of f or est

c ar b o n, t h e p ot e nti all y l ar g e a n d c h a n g e a bl e q u a ntit y of

c ar b o n i n s oil is n ot as i m m e di at el y a p p ar e nt. T o t h es e

p o ols m ust als o b e a d d e d c ar b o n ass o ci at e d wit h

h ar v est e d ti m b er a n d r et ai n e d i n t h e s h ort or l o n g t er m as

w o o d pr o d u cts a n d, e v e nt u all y, as a c o ntri b uti o n t o t h e

l a n dfill c ar b o n p o ol. T h e f oll o wi n g s e cti o ns d e al wit h

e a c h of t h es e c ar b o n p o ols i n t ur n.

Tr e e s a n d f o r e s t s a s c oll e c t o r s of C O 2

Li k e all gr e e n pl a nts, tr e es assi mil at e C O 2 f r o m t h e

at m os p h er e t hr o u g h t h e pr o c ess of p h ot os y nt h esis. T h e

si m pl e s u g ar m ol e c ul es t h at ar e i niti all y f or m e d fr o m C O 2

t h e 1 9 9 0s, t err estri al v e g et ati o n is b eli e v e d t o h a v e

a bs or b e d b et w e e n 2 a n d 3 Gt C p er y e ar at t h e s a m e ti m e.

T his is p artl y as a r es ult of hi g h er C O 2 c o n c e ntr ati o ns i n

t h e at m os p h er e e n h a n ci n g, or f ertilisi n g, pl a nt

p h ot os y nt h esis, b ut als o r efl e cti n g t h e r e- gr o wt h of

i nt e nsi v el y m a n a g e d or pr e vi o usl y cl e ar e d f or ests i n s o m e

r e gi o ns. T h es e o bs er v ati o ns h a v e l e d t o t h e vi e w t h at

c ert ai n f or estr y-r el at e d m e as ur es c o ul d m a k e a si g nifi c a nt

c o ntri b uti o n t o t h e miti g ati o n of cli m at e c h a n g e t hr o u g h:

• h alti n g d ef or est ati o n;

• s u p p orti n g r ef or est ati o n a n d aff or est ati o n;

• c o ns er vi n g or e n h a n ci n g t h e c o ntri b uti o n m a d e b y

e xisti n g f or ests t hr o u g h f or est m a n a g e m e nt.

T h e m ai n p ur p os e of t his I nf or m ati o n N ot e is t o d es cri b e

t h e p ot e nti al e xt e nt of t his c o ntri b uti o n, pri m aril y wit h

r el e v a n c e t o t h e U K. It pr o vi d es a g e n er al b a c k gr o u n d t o

t h e s ci e n c e ass o ci at e d wit h f or ests a n d c ar b o n a n d e x pl ai ns

t h e r ol e of f or ests a n d f or est pr o d u cts i n cl u di n g bi o e n er g y

a n d w o o df u el. O n g oi n g i nt er n ati o n al n e g oti ati o ns ar e

r ef err e d t o, pr o vi di n g s o m e c o nt e xt f or t h e i nt er est i n t h e

m a n a g e m e nt of f or ests a n d f or est pr o d u cts as m e a ns t o

h el p miti g at e cli m at e c h a n g e. Fi n all y, o pti o ns ar e

dis c uss e d f or m a n a gi n g f or ests i n t h e U K t o m a xi mis e

t h eir c o ntri b uti o n t o r e d u ci n g n ati o n al G H G e missi o ns.

At m os p h er e

D e c a y 5 4

R e s p ira tio n6 0

P h o to s y n th e s is 1 2 0 .4

F ire 4

R esp

iratio

n 9 0 .3

Abso

rptio

na n

dp h o to sy n th e sis 9 2

O il C o a l a n d p e a t

F ossil f u el us e

C o m b usti o n6. 3

B ur ni n g & d e c a y1

F ossil f u els

4 0 0 0

W o o d pr o d u cts & f u el w o o d

5

T e rr estri al v e g et ati o n1 4 0 0

5 5 0

7 5 0

+ 3. 2

O c e a ns

3 9 0 0 0

R o c k

6 5 0 0 0 0 0 0

1. 1

Fi g u r e 2

T h e gl o b al c ar b o n b u d g et f or t h e 1 9 9 0s. T h e n et i n cr e as e of 3. 2 Gt C yr - 1 i nt o t h e at m os p h er e is s m all c o m p ar e d t o t h e t ot al c ar b o n st o c ks i n v e g et ati o n,s oil, r o c ks a n d t h e o c e a ns, a n d als o w h e n c o m p ar e d t o t h e q u a ntit y of c ar b o n i n t err estri al v e g et ati o n al o n e. A si m plifi e d s u m m ar y of t h e gl o b al c ar b o nb al a n c e is als o gi v e n.

C o nt ri b uti o n E mitt e d ( s o u r c e )

A b s o r b e d ( si n k )

B ur ni n g f ossil f u els 6. 3

L a n d- us e c h a n g e ( pri m aril y d ef or est ati o n) 1. 6

E n h a n c e d v e g et ati o n gr o wt h 3. 0

O c e a n – at m os p h er e e x c h a n g e 1. 7

T ot al 7. 9 4. 7

B al a n c e 3. 2

F or e str y C o m mi s si o n

A R C HI V E

All st o c ks (fi g ur es i n r e d) a n d e x c h a n g es (fi g ur es i nbl a c k) of c ar b o n ar e s h o w n i n u nits of gi g at o n n es(t h o us a n d milli o n t o n n es) of c ar b o n p er y e ar.

3

ar e t h e n c o m bi n e d t o pr o d u c e c ell ul os e, as w ell as li g ni n

i n t h e c as e of w o o d y pl a nt or g a ns. M u c h of t h e c ar b o n

t h at is assi mil at e d t hr o u g h p h ot os y nt h esis is r el e as e d

a g ai n as C O 2 t hr o u g h r es pir ati o n – t h e e n er g y c osts

ass o ci at e d wit h gr o wt h a n d m ai nt e n a n c e of li vi n g

m at eri al. T h e r e m ai ni n g c ar b o n is all o c at e d t o l e af, r o ot,

s e e d, w o o d a n d br a n c h bi o m ass. At a n a n n u al ti m es c al e,

t h e c ar b o n ass o ci at e d wit h s h ort-li v e d c o m p o n e nts of

w o o dl a n d is r et ur n e d t o t h e at m os p h er e t hr o u g h

d e c o m p ositi o n, wit h o nl y a pr o p orti o n of fi x e d c ar b o n

b ei n g r et ai n e d i n t h e l o n g er t er m as w o o d. A t y pi c al

c ar b o n b u d g et is gi v e n i n Fi g ur e 3, b as e d o n m e as ur e d

a n d m o d ell e d el e m e nts of a l o wl a n d o a k w o o dl a n d

e c os yst e m i n s o ut h er n E n gl a n d.

Fi g u r e 3

A s u m m ar y of t h e c ar b o n e x c h a n g e (i n t o n n es of c ar b o n p er h e ct ar ep er y e ar) ass o ci at e d wit h t h e m ai n c o m p o n e nts of o a k w o o dl a n d( g e n er al yi el d cl ass 6 m3 h a - 1 yr - 1) at t h e Str aits E n cl os ur e fl u x st ati o n i nH a m ps hir e. F or c o m p aris o n wit h t h e v al u e f or i n cr e m e nt gi v e n h er e( 4. 0 t C h a- 1 yr - 1), a v al u e of 3. 8 t C h a- 1 yr - 1 w a s o bt ai n e d f or n ete c os yst e m e x c h a n g e i n 2 0 0 0 usi n g e d d y c orr el ati o n (s e e Fi g ur e 8).N ot e t h at t h e q u a ntit y of c ar b o n a d d e d t o t h e s oil c ar b o n st o c k o n a na n n u al b asis is u n k n o w n a n d, if si g nifi c a nt, w o ul d r e d u c e t h e q u a ntit yof c ar b o n l ost t hr o u g h litt er d e c a y.

Fi g u r e 4

A n e x a m pl e of c ar b o n a c c u m ul ati o n i n a n e wl y cr e at e d st a n d of tr e es.F o ur p h as es of gr o wt h or c ar b o n s e q u estr ati o n c a n b e s e e n: a: est a blis h m e nt p h as e; b: f ull- vi g o ur p h as e; c: m at ur e p h as e; d: ol d-gr o wt h p h as e. L o o ki n g o v er s e v er al d e c a d es it is e vi d e nt t h at, f oll o wi n ga n i n cr e as e i n c ar b o n st o c ks o n t h e gr o u n d d u e t o t h e i niti alest a blis h m e nt of t h e st a n d, c ar b o n st o c ks n eit h er i n cr e as e n or d e cr e as eb e c a us e a c c u m ul ati o n of c ar b o n i n gr o wi n g tr e es is b al a n c e d b y l oss esd u e t o n at ur al dist ur b a n c es a n d d e c a y of d e a d w o o d o n sit e. T w oe x a m pl es of c ar b o n d y n a mi cs wit h diff eri n g d e gr e es of n at ur aldist ur b a n c e a n d t h us l o n g-t er m e q uili bri u m c ar b o n st o c ks ar e ill ustr at e d.T h e l o n g-t er m a v er a g e st o c k, as b as e d o n t h e t w o dist ur b a n c e s c e n ari os( d as h e d li n es), is esti m at e d t o li e i n t h e r a n g e 1 7 0 t o 2 2 0 t C h a - 1. T h eesti m at es u p t o a g e 8 0 ar e r e pr es e nt ati v e of a st a n d of g e n er al yi el d cl ass1 2 Sit k a s pr u c e a n d h a v e b e e n m a d e usi n g t h e C A R BI N E c ar b o na c c o u nti n g m o d el ( T h o m ps o n a n d M att h e ws, 1 9 8 9; M att h e ws, 1 9 9 1).

T h e d y n a mi c s of c a r b o n a c c u m ul a ti o n i nw o o dl a n d

T h e p att er n of a c c u m ul ati o n of c ar b o n i n w o o d y bi o m ass

o v er t h e lif e c y cl e of a st a n d of tr e es r efl e cts ti m b er i n cr e-

m e nt, si n c e t h e dr y w ei g ht of w o o d c o m pris es 5 0 % c ar b o n,

a n d st e m w o o d c o m pris es t h e b ul k of tr e e bi o m ass. M o d els

or t a bl es pr o vi di n g f or e c asts of ti m b er gr o wt h a n d yi el d of

f or ests c a n t h us b e us e d t o esti m at e c ar b o n st o c ks a n d

a c c u m ul ati o n r at es (s e e p a g e 8, ass essi n g c ar b o n st o c ks,

si n ks a n d s o ur c es), alt h o u g h o nl y a r a n g e of m a n a g e m e nt

o pti o ns ar e c o v er e d i n p u blis h e d f or est yi el d t a bl es. A n al ys es

f or aff or est ati o n b as e d o n yi el d m o d els a n d t a bl es f or e v e n-

a g e d st a n ds h a v e s h o w n t h at t h e r at e of c ar b o n a c c u m u-

l ati o n is r el ati v el y l o w d uri n g t h e st a n d est a blis h m e nt p h as e

( a n d m a y e v e n b e n e g ati v e as a r es ult of c ar b o n l oss fr o m

v e g et ati o n a n d s oil ass o ci at e d wit h gr o u n d pr e p ar ati o n).

T his is f oll o w e d b y t h e f ull- vi g o ur p h as e, a p eri o d of

r el ati v el y r a pi d u pt a k e, w hi c h l e v els off as t h e st a n d r e a c h es

t h e m at ur e p h as e, a n d t h e n f alls, as s h o w n i n Fi g ur e 4.

Ulti m at el y, f or est st a n ds a p pr o a c h t h e ol d- gr o wt h p h as e

d uri n g w hi c h t h e y ar e i n l o n g-t er m e q uili bri u m wit h l oss es

of c ar b o n t hr o u g h m ort alit y a n d dist ur b a n c e b al a n ci n g

a n y a d diti o n al gr o wt h. S m all a m o u nts of c ar b o n m a y

c o nti n u e t o a c c u m ul at e i n t h e s oil, wit h t h e ti m e f or s oil

c ar b o n t o r e a c h e q uili bri u m b ei n g m u c h l o n g er t h a n t h at

f or f or est bi o m ass. T h e a c c u m ul ati o n of c ar b o n b y f or est

st a n ds is oft e n r ef err e d t o as c ar b o n s e q u estr ati o n; s e e t h e

d efi niti o ns b o x o v erl e af f or a m or e d et ail e d dis c ussi o n.

N e w litt er i n p ut fr o m:

F oli a g e 2. 0S e e ds 0. 6W o o d 1. 1U n d erst or e y 0. 4

T ot al 4. 1

D e c a y of litt er:

4. 1 – ? = ?

R es pir ati o n:

5. 9

I n cr e m e nt ( n et bi o m ass a c c u m ul ati o n) t o:

St e m w o o d 2. 7R o ots 0. 5Br a n c h es 0. 5S hr u bs 0. 3

T o t al 4. 0

N et pri m ar y pr o d u cti vit y( n e w bi o m ass):

1 4. 0 – 5. 9 = 8. 1

N et r e m o v al of C O 2fr o m at m os p h er e:

1 4. 0 – 5. 9 – 4. 1 = 4. 0

N et i n cr e as e i n s oil c ar b o n:

?

Gr oss pri m ar y pr o d u c-ti vit y ( p h ot os y nt h esis):

1 4. 0

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0

St a n d a g e ( y e ars)

Carbon

stoc

k in

stand

(tC

ha-1 )

a

b

c d

F or e str y C o m mi s si o n

A R C HI V E

4

The maximum rate of carbon accumulation during thefull-vigour phase of fast growing stands in the UK is about10 tC ha-1 yr-1, although a realistic average over a fullcommercial rotation may be no more than 3 tC ha-1 yr-1.The maximum potential for carbon accumulation in UKwoodland, and by implication the quantity of CO2

removed from the atmosphere over the life cycle of astand of trees, is approximately 200 tC ha-1. Thisrepresents the carbon reservoir in old-growth woodlandallowing for periodic disturbance events. Managed fellingoccurs more frequently than disturbance by naturallyoccurring events and, generally, commercially managedstands do not reach old-growth conditions. As a roughguide, commercially managed stands can be assumed toaccumulate up to approximately 100 tC ha-1 averagedover a number of rotations, as illustrated for a specificexample in Figure 5. The situation for stands managedaccording to continuous cover methods is likely to besimilar to that described above for even-aged stands. Forexample, the dynamics of carbon absorption in newlycreated continuous cover woodlands is likely to be verysimilar to that for even-aged stands up to the maturephase. From this point on, the pattern of carbon stocksmaintained over time will depend on the specificsilvicultural regime being practised. In a system based onpatch felling or tree selection and regeneration, individual

0

50

100

150

200

250

0 20 40 60 80 100 120 140 160 180 200

Stand age (years)

Car

bon

stoc

k in

sta

nd (

tC h

a-1 )

Figure 5

An example of the carbon stocks associated with above-groundbiomass of an even-aged stand of trees, felled and replanted on a 50-year rotation in order to maintain a high growth rate in the stand. Onthe timescale of many rotations, carbon stocks in the standing treesfollow a cycle between close to zero and about 160 tC ha-1. The long-term average carbon stock maintained on the site is approximately 70tC ha-1 as marked by the dashed line. The estimates up to age 50 arerepresentative of a stand of yield class 12 Sitka spruce and have beenderived using the CARBINE carbon accounting model (Thompson andMatthews, 1989; Matthews, 1991).

Carbon sequestration, sinks and sources in vegetation – some definitions

The accumulation of carbon by forest stands is often referred to as carbon sequestration. In legal terms, the verb to sequester isdefined as to seize temporary possession (of something). This makes a good analogy with the pattern of carbon dynamicsdescribed, highlighting four important features:

• Individual atoms of carbon are continually being exchanged between the atmosphere and a forest stand – i.e. an individualatom is only captured from the atmosphere temporarily.

• Over the lifetime of a forest stand, more carbon atoms are captured than are released so there is net accumulation of carbon inthe forest.

• Carbon is only accumulated by a forest up until the point when equilibrium is reached, so that the quantity of carbonaccumulated is strictly finite.

• The accumulation of carbon by a forest is reversible, with carbon being returned to the atmosphere through dieback, decayand burning of wood if the forest stands are not maintained.

The carbon balance of a forest needs to take into account the exchanges of carbon between the atmosphere and all the carbonpools associated with the forest. The sum of all the carbon pools is known as the carbon stock of the forest. A carbon balance isoften described as representing a sink (resulting in carbon sequestration) if there is a net transfer of carbon (in the form of CO2)from the atmosphere to one (or any combination) of the carbon pools associated with the forest and the wood products chain.When a forest is described as a carbon source there is a net transfer of carbon to the atmosphere.

Individuals, organisations and countries may consider using carbon sequestration to compensate for emissions of CO2 associatedwith a particular activity. If they are successful, the activity may be claimed to be carbon-neutral. Initiatives based on this approachmay be referred to as carbon-offset or carbon-neutrality schemes. Quantities of sequestered carbon claimed against CO2 emissionsare sometimes called carbon credits, and the buying and selling of these credits may be referred to as carbon trading.

patches or trees may follow a pattern similar to that forcarbon stocks shown in Figure 5. However, averaged overthe stand, the pattern of carbon stocks may look more likeone of the patterns illustrated in Figure 4.

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The lower rate of carbon sequestration in the mature phasecompared to the full vigour phase has led some comment-ators to infer that young, faster-growing trees are ‘better’for carbon sequestration. However, this interpretation isnot supported by the long-term average carbon stocks ofold-growth woodland and young plantation forests shownin Figures 4 and 5. These figures illustrate how the establish-ment of forest stands can increase on-site carbon stocks,whether or not periodic harvests are carried out. However,the level of the long-term carbon stock depends on thebalance between the impacts of harvesting events and therate of forest regeneration. A comparison of the two figuresalso illustrates how periodic harvesting of old trees andreplacement with young trees can lead to a lower long-term carbon stock than would be the case if harvestingwas avoided.

The quantity of carbon captured and retained by forests isgenerally significantly higher than the above-ground carbonreservoirs associated with other vegetation types, forexample, perennial grassland (typically 5 tC ha-1) andheathland (typically 10 tC ha-1), and underlies theperceived importance of the role of forests in the globalcarbon budget. Ground vegetation, particularly wherethere is a well-developed understorey, can contribute towoodland carbon while, in some stands, the carbonassociated with accumulating brash, leaf, seed and woodlitter may also represent a significant carbon pool. Thelarge amount of litter transferred to forest soils generallyresults in an increase in the carbon content of those soils,and this has been well demonstrated in studies ofwoodland colonisation of tilled farmland, an example ofwhich is shown in Figure 6.

Forest soils as carbon reservoirs

Forest soils can contain more carbon than the treescomprising the forest – particularly many of the peat-based soils common in the uplands. Table 1 gives averagesoil carbon contents for a variety of land cover types inBritain. Two main reasons account for the variation insoil carbon content for the different land-use classes.Firstly, in some parts of the UK, many of the commercialforests have been planted on upland soils which alreadycontained high levels of organic matter. For example,many forests in Scotland were planted on moorland whichgenerally has a high soil carbon content. The high carbondensity of forest soils in Scotland is, therefore, notnecessarily a result of afforestation but may reflect thecarbon density of the previous land cover. Secondly, soilcarbon density is generally higher under forests and semi-natural vegetation than under more intensive land-usessuch as arable agriculture.

5

Figure 6

The accumulation of carbon stocks in soil in a field at Rothamsted,Hertfordshire. The field was initially managed for arable agriculturalproduction but ‘set aside’ in 1875 to create a ‘wilderness’. Over thepast 100 years or more, the field has regenerated naturally into an areaof mixed broadleaf woodland. Redrawn after Poulton (1996).

0

10

20

30

40

50

60

70

1883 1907 1965 1987

Soil

orga

nic

carb

on (

tC h

a-1 )

Land cover England Scotland Wales NorthernIreland

Semi-natural 487 1048 305 551

Woodland 217 580 228 563

Arable 153 156 93 151

Pasture 170 192 200 178

Other 33 141 43 102

Table 1

Average soil carbon stocks (tC ha-1) for different land cover types in theUK, based upon information provided by the National Soil ResourcesInstitute, the Macauley Land Use Research Institute and Queen’sUniversity Belfast (Milne, 2001). Semi-natural vegetation includesheathland, moorland and scrub vegetation as land cover types.

Changes in woodland soil carbon depend on a balancebetween the accumulation of dead biomass, itsincorporation into the soil and losses due to respirationand decay. The rates of litter input and decomposition canbe influenced by management practice, while any changein climate, particularly rainfall patterns and temperature,will also affect the rate of carbon loss or gain in woodlandsoils. Any soil disturbance associated with forestmanagement may release carbon to the atmosphere, andshould be minimised to optimise soil carbon stocks.

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Peatlands, greenhouse gases and forestry

The practice of draining peat-based soils, either prior to oras a result of afforestation, has led to debate over thepotential for carbon losses. When these soils are drained,either artificially or through trees using more water thanthe previous land-cover and lowering the water table,oxidation of the peat increases and considerable quantitiesof carbon can be lost to the atmosphere as CO2. One studyhas indicated that following planting on an upland peatsoil, more carbon may be lost than gained by a stand ofSitka spruce for a period of 8–12 years. However, recentresearch suggests that growth of grass and other vegetationon newly afforested peatland can compensate for this lossof carbon and so net uptake may begin sooner than prev-iously thought. The picture becomes even more complicatedif other GHGs are taken into account. Although drainagemay lead to an initial increase in CO2 emissions, naturallyoccurring methane emissions from the peat fall at the sametime. Methane (CH4) is a more potent GHG than CO2, andcalculations indicate that over some timescales tree plantingcould be beneficial in terms of GHG balance of peat.Further complications arise due to a probable contributionfrom increased emissions of another GHG, nitrous oxide(N2O), highlighting the complex issues surrounding GHGbalance of peatlands and the need for further research.The scientific uncertainty associated with afforestation ofpeatlands and their GHG emissions indicates that there isnot a strong case for the conversion of peatland towoodland when the sole objective is carbon sequestration.

Wood products

Wood products such as paper, joinery and timber used inthe construction industry represent a secondary carbonreservoir associated with the management of forests.However, for wood products to sequester carbon, the sizeof this reservoir must grow – simply replacing old productswith new does not increase the amount of carbon in thewood products pool. The main component contributing tothis pool is timber associated with the constructionindustry (Figure 7). However, it has been estimated thatthe carbon in wood disposed (currently and historically)to landfill in the UK constitutes an even larger pool thanthat associated with wood products in use. At present, thewood products pool in the UK (excluding landfill) isestimated to contain 80 MtC (just over half that containedin the biomass of UK forests) and to be increasing at arate of 0.44 MtC yr-1. These numbers bear comparisonwith those for living biomass in UK forests. This is aconsequence of the relatively small area of forests in theUK compared to the volumes of wood consumed and the

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Estimated carbon stocks in wood products in primary and secondary usein the UK over the period 1965 to 2000 showing the contributions madeby different types of product. Carbon stocks in wood products disposedof to landfill are not included. Note that currently, approximately 90% ofthe harvested wood making up UK wood products is imported fromoutside the UK. Redrawn from Alexander (1997).

dominant contribution (about 85%) from imported timber.At the global scale, the contribution from harvested woodproducts to the forest carbon sink is much less important.The use of harvested wood products is one of the activitiesthat may in future form part of national carbonaccounting. As explained later (page 10), wood productshave a broader role in the carbon balance beyond simplycontributing to forest carbon stocks.

BALANCING SOURCES AND SINKS:SIZING UP THE CHALLENGE

Many commentators believe that climate change and theglobal carbon budget are key to the future of the globalenvironment and its potential effect on mankind. In recog-nition of the importance of managing emissions of GHGs,a number of both political and practical initiatives have beenproposed, with the intention of minimising GHG emissionsand maximising carbon sinks. Some of these proposalsinclude important roles for forestry. The followingsections describe these initiatives and assess the extent ofthe contribution that woodlands in Britain might make.

International negotiations and thepolitical context

The contribution that the UK can make to globalemissions reductions takes place within an international

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framework. The development of this framework originatedin 1988 when the World Meteorological Organisation(WMO) and United Nations Environment Programme(UNEP) established the Intergovernmental Panel onClimate Change (IPCC) following concerns over thepossible impacts of climate change. The remit of the IPCCis to assess available scientific, economic and technicalmaterial relating to human-induced climate change, butnot to carry out research or monitoring activities itself.Three assessment reports have been produced (1990, 1995and 2001), the first of which led to the adoption of theUnited Nations Framework Convention on ClimateChange (UNFCCC) in 1992. To date, it has been ratifiedby over 180 governments. Article 2 of the UNFCCC statesas a primary objective: to achieve stabilisation ofgreenhouse gas concentrations in the atmosphere at a levelthat would prevent dangerous anthropogenic interferencewith the climate system. Negotiations through theframework of the UNFCCC subsequently resulted in thedrafting of the Kyoto Protocol (see box below) on GHGemissions reductions in 1997, which has so far beenratified by over 100 governments.

The potential contribution of UK forests

The forest estate of the UK covers an area of 2.8 millionhectares, or 11.6% of the land surface, equivalent to anarea about the size of Wales. The biomass which constitutesthese forests contains about 150 MtC, which is roughly equalto one year of CO2 emissions from burning fossil fuels andcertain industrial processes in the UK, as shown in Table 2.For the UK to become carbon-neutral (in terms of CO2

emissions at current rates) through afforestation alone, itwould be necessary to create 50 million hectares of forestthat maintained an average rate of carbon sequestration ofaround 3 tC ha-1 yr-1 over the period from establishment to theold-growth phase. This is approximately double the land areaof the UK. Moreover, once the forests attained the old-growthphase (perhaps after 100 years) they would stop acting as acarbon sink. Clearly, forest carbon sequestration alone cannotbe used to offset GHG emissions from fossil fuel consumptionand industrial processes associated with contemporary waysof living and working. However, the carbon sink associatedwith UK forests could make a useful contribution as policiesand initiatives are developed to reduce GHG emissions.

The Kyoto Protocol – what is it?

The Kyoto Protocol is an agreement among governments that sets targets for reductions in GHG emissions and specifies the

mechanisms that may be used to achieve them. These include reductions in the consumption of fossil fuels and options involving

carbon sequestration through forestry activities. The IPCC is continuing to develop a framework and methodologies by which

governments can realise and report their national GHG emissions reductions. There are three Articles of the Protocol directly

relevant to forests and forest management:

• Article 2.1 calls for the ‘protection and enhancement of sinks and reservoirs of greenhouse gases’ and the ‘promotion of

sustainable forest management practices, afforestation and reforestation’.

• Article 3.3 states that ‘the net changes in greenhouse gas emissions by sources and removals by sinks resulting from direct

human-induced land-use change and forestry activities limited to afforestation, reforestation and deforestation since 1990,

measured as verifiable changes in carbon stocks in each commitment period, shall be used to meet commitments under the

Protocol’. By any generally accepted interpretation, restricting the accounting to afforestation, reforestation and deforestation

activities since 1990 (sometimes referred to as the ‘Kyoto forests’) means that only a very small percentage of the world’s

forests are covered by Article 3.3. The Protocol further stipulates that greenhouse gas emissions by sources and removals by

sinks associated with these activities shall be reported in a ‘transparent and verifiable manner’ (see page 8, assessing carbon

stocks, sinks and sources).

• Article 3.4 allows countries to account for carbon stock changes and non-CO2 GHG emissions arising from other activities

including the management of forests existing before 1990. The magnitude of any carbon sequestration due to human

intervention must be verifiable. The Kyoto Protocol places restrictions on the extent to which carbon sequestration of this kind

can be claimed, and details of how this would be monitored and reported are currently being elaborated by the IPCC.

The Kyoto Protocol also makes provision for industrialised countries (so-called ‘Annex I’ countries) to claim carbon credits in ‘non-

Annex I’ countries through specific projects aimed at reducing GHG emissions (the Clean Development Mechanism: Article 12) or

through collaboration with other Annex I countries such that emissions reduction credits can be transferred (Joint Implementation:

Article 6).

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Under the Kyoto Protocol, the UK is committed to reducingCO2 emissions from the rate reported for 1990 of 165MtC yr-1 by 12.5 % (or 20.6 MtC yr-1) during the five-yearperiod from 2008 to 2012. A larger, voluntary target hasbeen adopted within the UK to reduce emissions belowthe 1990 level by 20% (about 33 MtC yr-1) by 2010. TheUK’s Climate Change Programme sets out measuresacross the economy intended to make this goal achievable.It has been estimated that carbon sequestration throughafforestation in the UK accounted for some 0.4 MtC yr-1

in 2001, rising to a value of around 2.9 MtC yr-1 if thecontribution of forests planted before 1990 (‘non-Kyotoforests’) is included. Projections to 2010 are for thesevalues to rise to 0.7 and 3.2 MtC yr-1, respectively,representing a contribution of either 3.4 % or 15.5 % tothe emissions reduction target set for the UK in the period2008 to 2012 under the Kyoto Protocol. To place theUK’s role in climate change mitigation in a global context,the total emissions reductions negotiated under the KyotoProtocol in 1997 amounted to 208 MtC yr-1.

Carbon neutrality, carbon offset andcarbon trading schemes

A number of schemes are currently operating which offerindividuals and organisations the opportunity to havetrees planted on their behalf with the aim of offsettingemissions of GHGs. An estimate is made of the carbon

sequestered over the lifetime of the woodland and this ismarketed as a carbon credit to offset emissions resultingfrom specified activities of an individual or organisation.Concerns have been raised over schemes that award creditsin advance of the carbon being sequestered. There is also aquestion mark over the long-term future of woodlandscreated for this purpose, because of uncertainties overfuture land management or the occurrence of naturalevents such as storms and droughts or the effects ofclimate change itself. Availability of land for afforestationlimits the potential for individuals or organisations to takeup these schemes, both in the UK and globally. One of theexamples most frequently encountered is that of ‘carbon-neutral car use’. As a rough guide, the carbon sequesteredby half a hectare of woodland over one rotation cancompensate for the CO2 emissions associated with car fuelconsumption during an average driver’s lifetime: thissounds a small area, but it should be borne in mind thatwith 30 million registered drivers in the UK, three-quarters of the land area of the nation would have to beafforested to make car use alone carbon-neutral.

ASSESSING CARBON STOCKS,SINKS AND SOURCES

Any framework for negotiating emissions reductions ortrading carbon credits must be on the basis ofscientifically robust, verifiable measurements. There aretwo principal ways by which the sizes of carbon pools orrates of carbon sequestration are commonly measured.

Measurement of stock change by periodicinventory

The simplest method of assessment involves measuring thedifference in carbon stocks between two points in time.Conventional forest mensuration methods are used eitherto measure or model timber volumes, which are then con-verted to dry weight by reference to tables of wood specificdensity, generally in the range of 0.33 to 0.45 t m-3 for soft-woods and 0.49 to 0.56 t m-3 for hardwoods in the UK.The carbon content (typically 0.5 tC t-1) is then used toconvert dry weight to carbon. Estimates derived in this wayrepresent quantities of carbon in the stemwood of trees,either standing or harvested as appropriate. In order toaccount for carbon in non-stem components as well as stem-wood, the estimates are increased by a factor known as a‘total:merchantable’ ratio or ‘expansion factor’. The valueof this factor depends greatly on tree species, stand age,management and environmental conditions. The range ofvalues published for mature trees varies between 1.3 and 1.8.

Source

UK carbon emissions (MtC yr-1)

1990 2001

Emissions Removals Emissions Removals

Roadtransport fuelconsumption

31.8 33.6

Energyindustries fuelconsumption

62.2 54.3

Other fuelconsumption

61.1 60.6

Industrialprocesses

3.9 3.2

Land-usechange andforestry

5.3 –2.9 4.0 –3.2

Wasteincineration

0.5 0.4

Total 164.8 –2.9 156.1 –3.2

Table 2

Comparison of UK carbon emissions inventories for the years 1990 and2001. Values are for CO2 emissions only (i.e. not counting other GHGs)and are expressed as million tonnes of carbon per year. The land-usechange and forestry inventory includes entries for both emissions andremovals, reflecting carbon losses associated with land-use change onone hand and vegetation growth on the other. Source: NationalEmissions Database, NETCEN.

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In this inventory-based accounting system, leaf biomass,ground vegetation and litter are often not included. Thecarbon content of the soil, although of great importance,has seldom been included because of difficulties indefining and carrying out cost-effective assessments of soilcarbon. Moreover, stock changes that may be small incomparison to total soil carbon stocks are difficult toidentify, particularly when uncertainties associated withthe measurements are considered. An alternative methodto account for changes in soil carbon is to combineinventories of carbon in forest vegetation with estimatesof soil carbon produced by models of soil carbondynamics. Depending on the purpose of the inventory,carbon stocks or stock changes in harvested woodproducts may or may not be assessed. Accounting forcarbon stocks in wood products is impossible forindividual stands of trees because of uncertainty over thelong-term fate of harvested wood once it has left theforest. At a district, national or global scale, someassessment of the size of (and changes to) the woodproducts pool may be possible by reference to relevantindustrial and domestic statistics.

Measurement of flux

An alternative method of assessment is known as the flux-based approach. This approach directly measures the netflow of carbon into or out of the forest. Technology hasbeen developed so that it is now possible to continuouslymonitor carbon exchange between all the carbon pools ina forest ecosystem and the atmosphere (see Figure 8),although the small amount of carbon that may be lost froma stand as particulate and dissolved organic carbon in rain-water drainage and runoff is not measured. Rather moreimportant is that losses of forest carbon due to harvestingcannot be measured directly. The advantage of the flux-based approach is that a net ecosystem flux is measured,accounting for all carbon pools, including deadwood andlitter and other fractions which prove difficult to measureusing stock-change methods. The major drawback of theapproach is its cost, and thus the small number of fluxstations that have been established to date. In the UK,there are only three long-term flux monitoring stations inforests, while across Europe, there are more than 30 suchsites. The choice of site is also limited by area andtopography and the measurements are only representativeof the species, site and growth stage under investigation.As with the stock-change approach, carbon in the woodproducts pool associated with a specific stand cannot beaccounted for.

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The choice of approach

The stock-change and flux-based approaches are best appliedat different scales of space and time. The best method touse depends mainly on the objective of the assessmentsbeing made and, crucially, on the geographical scale thatneeds to be considered and on the resources available. Aninventory-based approach, particularly if used to assesscarbon stocks or sequestration in woody biomass only,can be used to cover large land areas and a variety ofspecies and site conditions. This approach could also bebased upon existing forest inventory networks such as theUK permanent sample plot network and the EU and ICP(Forests) Forest Monitoring Programme. The flux-basedapproach works best at providing information on short-term variations in the magnitude of the carbon sink and inquantifying net carbon exchange in forest systems wherethe individual pools are difficult to measure. A further roleis to provide data for the development and calibration ofprocess-based forest growth models. However, the mostimportant role of flux-based assessments is to provide acrosscheck for inventory methods across different forestrysystems and to provide essential information on how theenvironment, particularly climate, may modify rates ofcarbon exchange. A combined approach using themethods described above, together with remote sensingtechnology (satellite imagery and aerial photography),probably represents the most robust and cost-effective wayto monitor carbon stocks and stock changes.

Figure 8

Measurements of net ecosystem flux at a short time resolution can pro-vide verification of inventory-based carbon stock assessments, includingan analysis of changes to the magnitude of pools which are otherwisedifficult to measure. Measurements are made continuously from a towerabove the canopy at highly instrumented sites. Technical details of themethodology are available from www.ierm.ed.ac.uk/research/edisol.

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THE CONTRIBUTION FROMFOREST MANAGEMENT – THESUM OF THE PARTS, OR MORE?

Earlier sections of this Information Note have explainedthe process of carbon sequestration in the key componentsof forest ecosystems and their associated carbon pools. Forsimplicity, these pools may be defined as above- and below-ground living biomass, dead biomass, soil carbon and thecarbon associated with the wood products pool in its manyforms. However, the potential for forestry activities toreduce or offset emissions of GHGs is not simply the sumof the impacts on these carbon pools. There is another, oftenpotentially very important impact on the carbon balancedue to forestry activities arising from opportunities to usewood products to reduce the consumption of fossil fuels.

Sustainably harvested wood can substitute directly for fossilfuels in the form of renewable wood fuel (or ‘bioenergy’),or indirectly as renewable wood products replacing, whereappropriate, materials such as concrete and steel whichinvolve high fossil fuel consumption in their production.These two roles of wood products and their potential impacton the carbon balance of woodland are illustrated byexamples in Figures 9 and 10. As a consequence of thisimportant dimension to the carbon balance of forestrysystems, maximising the carbon benefits of an area ofwoodland is not always a simple question of leaving thetrees to grow and maximising the on-site carbon stock.

Figure 9

Illustration of the recycling of carbon as biomass accumulates in energycrops and forests and is consumed in a power station. a: CO2 is capturedby the growing crops and forests; b: carbon is retained in the biomassof the plants and trees; c: carbon in harvested biomass is transportedto the power station; d: the power station burns the biomass, releasingthe CO2 captured by the plants back to the atmosphere. Consideringthe process as a whole, there are no net CO2 emissions from burningthe biomass. In practice, an element of fossil fuel consumption is usuallyneeded for crop and forest management, biomass transport andprocessing and also for the running of the power station.

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A simulation of carbon stock changes in a forestry system together withcarbon emissions avoided through reduced fossil fuel consumptionarising from wood utilisation. The simulation was produced using thecarbon accounting model CO2FIX and represents a ‘typical’ commercial,even-aged stand of Norway spruce growing in Europe on a rotation of75 years and subjected to periodic thinning. In this example soilcarbon represents the largest contribution to the stock of carbon in theforest ecosystem in the timescale considered. The contribution madeby wood products harvested from the stand to overall carbon stocks isrelatively small compared to stocks in forest biomass and soil. Thecarbon stock in wood products harvested from the stand rises overseveral rotations to a maximum, after which no further accumulationtakes place. This is due to the relatively short lifetimes of wood productscompared to the rotation period of the stand from which they areharvested. However, as more and more wood products and bioenergyare harvested through thinning and felling activities, a succession ofopportunities arise to use harvested wood to avoid consumption offossil fuels. As a result, the magnitude of the emissions reductionthrough avoided fossil fuel consumption accumulates indefinitely, insharp contrast to the cycling of carbon stocks in the biomass, soil andwood products pools. By the end of the third rotation the emissionreduction achieved is similar in magnitude to the sum of carbon stocksin biomass, soil and wood products and in subsequent rotations willgreatly exceed them. Redrawn after Nabuurs (1996).

FOREST CARBON MANAGEMENT

Forest carbon management requires an in-depth assessmentof numerous factors including site conditions, potentialproductivity, vulnerability to natural events, proximity topoint of use and the local practicalities of the best andmost realistic options for end-use of harvested wood.Most of these considerations go beyond the commercial,social and environmental issues which currently form thebasis of conventional forest management plans in the UK. A number of options are available for maintaining orenhancing the carbon benefits of forests based on widelydiffering forest management systems and objectives. Anevaluation of these options for a stand, forest or region canbe based on a common set of principles, most importantly:

• Maintenance or enhancement of long-term on-sitecarbon stocks in the woodland itself.

• Minimisation of disturbance to litter and soil, to avoidcarbon emissions and soil degradation.

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high nutrient content of a significant proportion ofbranchwood compared with timber, often only a carefullyselected fraction of this material is harvested, primarilyavoiding the removal of foliage. As with carbon reservemanagement, there may be benefits associated with thewoodland in addition to its climate change mitigationrole. Carbon substitution (or displacement) managementis particularly well suited to even-aged forest stands withmoderate to high growth rates in localities with obviousopportunities for utilisation of harvested wood. Shortrotation coppice managed for bioenergy productionrepresents the ultimate expression of carbon substitutionmanagement. Stem quality may also require considerationwhen options are being evaluated, because it will have animpact on the potential to convert stemwood intodifferent products.

Selective intervention carbon management. This option issimilar to carbon reserve management but, in addition,there is low-level harvesting of certain trees to clearlydefined specifications in order to supply high-value nicheapplications. It is well suited to stands containing trees ofvariable quality where risk of significant naturaldisturbance is low and which may be some distance fromcentres of population or industry. Examples of this type ofmanagement include occasional tree harvests in stands tomeet a requirement for fuelwood in a small localcommunity and selective felling in continuous coverforestry systems to satisfy specialist timber markets.

As a general guide, selective intervention and carbonreserve management will usually result in higher long-termcarbon stocks within a given woodland ecosystem. On theother hand, only substitution and, to a lesser extent,selective intervention carbon management have the potentialto deliver long-term reductions in GHG emissions due tothe forestry system beyond the one-off increase in thecarbon pools associated with new or conserved woodland.

CONCLUSIONS: THE LIMITED BUTIMPORTANT ROLE OF FORESTRY

The world’s forests currently play an important role inminimising the extent and progress of climate change. Thecarbon that they contain would be enough to raiseatmospheric CO2 concentrations to well over 1000 ppmand with it a potentially catastrophic rise in temperatureof 5–8ºC. Maintaining global forests is therefore anessential element of any programme of measures tomitigate climate change. In the UK, our forest estatecovering about 12% of the land contains only as much

• Ensuring energy efficiency in woodland managementoperations and in the conversion of harvested trees toproducts.

• Matching timber and woodfuel production to possibleend-uses to achieve optimal utilisation of wood inreducing fossil fuel consumption.

• Harmonising carbon management with otherobjectives and with practical constraints.

Options for woodland management

Three contrasting options for woodland carbonmanagement, based on the principles described in theprevious section, are outlined below and an indication isgiven of their relative merits and potentials. These optionsare not prescriptive or exhaustive, but do indicate thegeneral range of approaches that are available for forestcarbon management. This section also shows how theabove principles may be used to evaluate managementoptions in terms of woodland carbon balance.

Carbon reserve management. This option is characterisedby minimal intervention, with a gradual long-term increasein carbon stocks. In addition to a climate change mitigationrole, carbon reserve management may also have significantamenity and biodiversity benefits, particularly if nativespecies are planted. Loss of carbon through fire, drought,floods or storm damage is minimised. For this reason it isnecessary to take account of wind-hazard, flood risk andclimate change predictions regarding the suitability of aparticular site-species combination to achieve this objective.At the same time, the use of fossil fuel during woodlandmanagement operations should be minimised so as not tonegate any enhancement of carbon stocks. Carbon reservemanagement is particularly well suited to forest standswith very low growth rates and poor stem quality, or inlocalities where there are limited opportunities for utilisationof harvested wood. The most extreme example of carbonreserve management might involve conservation of existingforest carbon stocks through avoidance of deforestation.

Carbon substitution management. This is characterised bycyclical changes in carbon density in the forest ecosystem(Figures 5 and 10), with maintenance of on-site carbonstocks being of secondary importance. This form ofcarbon management and its objectives are not far removedfrom the production forestry practised across most of theUK forest estate. Woody biomass is harvested as goodquality stemwood for use in product displacement andrenewable woodfuel (in the case of thinnings and harvestresidues). Soil disturbance following thinning or clearfellis minimised to limit carbon emissions. Because of the

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carbon as the nation emits in a single year from fossil-fuelburning. It would be impractical for the UK tocompensate for its GHG emissions through tree plantingalone, although carbon sequestration could be viewed asone of the environmental benefits associated with newwoodland creation. Harvested wood products can make asignificant contribution to climate change mitigationthrough their potential to substitute for fossil fuels, bothdirectly in the form of bioenergy, and indirectly throughproduct substitution. Where carbon-offset schemes oractivities aimed at claiming carbon credits are underconsideration, caution should be exercised assequestration through afforestation is finite, potentiallyshort-term and reversible, and verification procedures arenot well developed at present. On the other hand, anymeasures taken now to protect and expand forest areascan increase the biomass resource potentially available tofuture generations and, in principle, support any efforts tomove towards a ‘low-carbon’ economy.

REFERENCES AND USEFULSOURCES OF INFORMATIONALEXANDER, M. (1997).

Estimation of national carbon stocks and fluxes of wood basedproducts. MSc thesis, University of Surrey.

BROADMEADOW, M., ed. (2002). Climate change: impacts on UK Forests. Forestry CommissionBulletin 125. Forestry Commission, Edinburgh.

CANNELL, M.G.R. (1999). Growing trees to sequester carbon in the UK: answers to somecommon questions. Forestry 72, 237–247.

DEWAR, R.C. (1991). Analytical model of carbon storage in trees, soils, and woodproducts of managed forests. Tree Physiology 8, 239–258.

DEWAR, R.C. AND CANNELL, M.G.R. (1992). Carbon sequestration in the trees, products and soils of forestplantations: an analysis using UK examples. Tree Physiology11, 49–71.

FAO (2000). Forest resources of Europe, CIS, North America, Australia,Japan and New Zealand. UN/ECE and FAO, Geneva.

IPCC (2000). Land-use, land-use change and forestry, eds R.T. Watson, I.R.Noble, B. Bolin, N.H. Ravindranath, D.J. Verardo and D.J.Dokken. A Special Report of the IPCC. Cambridge UniversityPress, Cambridge.

IPCC (2001). Climate change 2001: the scientific basis, eds J.T. Houghton,Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden and D.Xiaosu. WG1 Report to the IPCC Third Assessment.Cambridge University Press, Cambridge.

MARLAND, G. AND MARLAND, S. (1992). Should we store carbon in trees? Special Issue: Natural sinks ofCO2, eds J. Wisniewski and A.E. Lugo. Water, Air, and SoilPollution 64, 181–195.

MATTHEWS, R.W. (1991). Biomass production and storage by British Forests. In: Woodfor energy: the implications for harvesting, utilisation andmarketing, ed. J.R. Aldhous. Proceedings of a discussionmeeting, Heriot-Watt University, Edinburgh, 5–7 April 1991.

MATTHEWS, R. AND ROBERTSON, K. (2002). Answers to ten frequently asked questions about bioenergy,carbon sinks and their role in global climate change. IEABioenergy Task 38: Greenhouse gas balances and bioenergysystems. Joanneum Research, Graz, Austria.

MILNE, R. (2001). Land use change and forestry: The 1999 greenhouse gasinventory for England, Scotland, Wales and Northern Ireland:1990, 1995, 1998 and 1999, eds A.G. Salway et al. NationalEnvironmental Technology Centre, AEA Technology, Harwell.

NABUURS, G.-J. (1996). Significance of wood products in forest sector carbon balances.In: Forest ecosystems, forest management and the globalcarbon cycle, eds M.J. Apps and D.T. Price. NATO ASI SeriesI, vol. 40. Springer-Verlag, Berlin, 245–256.

POULTON, P.R. (1996). Geescroft Wilderness, 1883–1995. In: Evaluation of soilorganic matter models using existing long-term datasets, edsD.S. Powlson, P. Smith and J.U. Smith. NATO ASI Series I, vol.38. Springer-Verlag, Berlin, 385–390.

SCHLAMADINGER B. AND MARLAND G. (2000). Land use and global climate change: Forests, LandManagement, and the Kyoto Protocol. Pew Center on GlobalClimate Change (www.pewclimate.org/projects/land_use.cfm).

THOMPSON, D. AND MATTHEWS, R.W. (1989). The storage of carbon in trees and timber. ResearchInformation Note 160. Forestry Commission, Edinburgh.

www.ipcc.chwww.ukcip.org.ukwww.futureforests.comwww.forestry.gov.uk/forest_research/carbonwww.eccm.uk.comwww.ierm.ed.ac.uk/research/edisolwww.forestry.gsi.gov.uk/economicswww.defra.gov.uk/environment/climatechangewww.nbu.ac.uk/ukcarbonwww.aeat.co.uk/netcen/airqual/statbasewww.dti.gov.uk/energy www.unece.org/trade/timber/frawww.shu.ac.uk/rru

Enquiries relating to this publication should be addressed to:

Mark BroadmeadowEnvironmental Research BranchForest ResearchAlice Holt LodgeWreccleshamFarnhamSurrey GU10 4LH

Tel: 01420 22255Fax: 01420 23653Email: mark.broadmeadow@forestry.gsi.gov.uk

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